Mental health problems cause a disproportionate burden of disability among children and youth compared to adults. Primary care plays an important role in efforts to prevent and intervene early in the course of child and adolescent mental health problems, but while research with adults has shown the feasibility of integrating mental health care into primary care settings there have been few studies among children and youth. Evidence remains lacking that integration is feasible in diverse settings, that it improves outcomes and that methods can be developed to address the multi-dimensional symptoms of emerging child/youth problems and their co-morbidity with developmental and parental disorders. Goals: The purpose of this application, in response to RFA-MH-15-330, is to test the effectiveness of adding a child/youth mental health component into an existing collaborative care program for adult mental health problems. The work will refine a framework for efficient cultural adaption and tailoring of an existing child/youth primary care mental health intervention and then test whether the tailored intervention results in improved child and parent outcomes. The work will also provide evidence about the mechanisms by which those outcomes are achieved and what factors influence uptake of the child/youth component by general practitioners (GPs). These results should be generalizable to other low and middle income countries and to underserved areas of the US where there are minimal child mental health resources and family physicians provide the bulk of medical care for children and youth. Methods: The planned work involves the adaptation/tailoring process followed by a hybrid effectiveness-implementation trial with 45 GPs already engaged in collaborative care for adults; the trial will study adding collaborative care for children ages 3-15. GPs will be randomly assigned, using a stepped wedge design, to begin 6-month control periods involving child mental health screening and referral. They will then receive child/youth training and begin second 6-month periods of screening plus ongoing coaching and booster sessions and collaborative management. Primary outcomes will be measured by recruiting and following for 6 months two cohorts of children/youth and their parents (one control, one collaborative care). Data collected from GPs, parents, youth, and the collaborative care data system will allow measurement of key moderators of implementation and effectiveness.